With the advent of smartphones, average data usage amount of mobile communication subscribers increases exponentially and contributes to the strong demand for high data rage. Typically, a high data rate can be achieved in such a way of increasing frequency bandwidth or improving frequency utilization efficiency. In the latter case, the current generation communication technologies have almost reached to the theoretical limit of the frequency utilization efficiency, it is difficult to further increase the frequency utilization efficiency through technical improvement. Therefore, a feasible approach to increase the data rate is to broaden the frequency band for data service. At this time, the available frequency band should be considered. Since the frequency band available for broadband communication in the bandwidth over 1 GHz is limited under the current frequency distribution policy, it is proposed to use the millimeter wave band over 30 GHz for wireless communication. However, such a high-frequency band communication has a drawback in that the signal attenuation increases significantly as the propagation distance increases. In detail, as the frequency increases, the propagation pathloss increases and the propagation distance decreases, resulting in reduction of the service coverage. One of the promising technologies to solve this problem is beamforming which concentrates transmission/reception power to narrow space to increase the transmission/reception efficiency.
FIG. 1 is a diagram illustrating a base station 100 and a mobile station 150 supporting the beamforming with array antennas. Referring to FIG. 1, the base station 100 is capable of transmitting data changing the direction of a downlink transmission beam 120 using the array antennas 110 and 112. Also, the mobile station 150 is capable of receiving data changing the direction of the reception beam 170.
In the communication system using the beamforming technique, the base station 100 and the mobile station 150 select the transmission beam 120 and the reception beam 170 showing the best channel condition among various transmission beam directions and reception beam directions to provide the data service. Such a procedure is applied identically to the uplink channel for transmitting data from the mobile station 150 to the base station 100 as well as the downlink channel for transmitting data from the base station to the mobile station.
It is assumed that a number of transmission beams directions of the base station 100 is N and a number of reception beams directions of the mobile station 150 is M. In this case, the simplest method for selecting the best downlink transmission/reception direction is that the base station 100 makes a trial to transmit a predetermined signal in N individual transmission directions at least M times and the mobile station 150 makes a trial to receive N transmission beams 120 using M reception beams 170.
In this case, the base station 100 has to transmit a specific reference signal at least N×M times and the mobile station 150 has to receive the reference signal N×M times to measure the received signal strength. The mobile station 150 may select the directions showing the highest measurement values as a combination of the best transmission beam direction and reception beam direction. The procedure in which the base station 100 transmits the signal at least one time in every available direction is called beam sweeping, and the procedure in which the mobile station 150 combines the best transmission and reception beam directions is called beam selection. This process of selecting the best downlink transmission beam and the beat downlink reception beam (hereinafter, referred to as Transmission beam and Reception beam) can be applied identically to the uplink transmission procedure for transmitting data from the mobiles station 150 to the base station 100.
In a normal cellular system, the base station 100 transmits the common control channels such as Synchronization Channel (SCH) and Broadcast Channel (BCH) within the whole coverage area of the base station 100. The base station 100 also has to receive the uplink access channel (Random Access Channel (RACH)) transmitted within the coverage area. In the communication system using the beamforming technology as shown in FIG. 1, the base station 100 has to transmit the above channels in every available direction at least once in the above-described beam sweeping manner to transmit the SCH and BCH within the whole coverage area of the base station 100. Also, in order to receive the uplink access channels transmitted within the whole coverage area of the base station 100, the base station 100 has to make a trial to receive the uplink access channel in every available direction at least once in the beam sweeping manner.